Tt no rcd at origin

[bigspark17]

200A 3phase tt system with metal clad control panel no rcd at origin 4pole main switch. I have always put rcd at origin but it has been put to me if you can prove a very stable low Ra (ze acceptable) rcd may omitted? What is an acceptable low level and how can you prove it will stay stable. No testing has been done yet was ,there to price another job this was me being nosey at origin. Thoughts please

 

[telectrix]

to omit the RCD, you need a PEFC large enough to operate the OCPD within specified timescale. a Ra value >1 ohm will not achieve this.

 

[Richard Burns]

A 200A BS88 will need about 1800A to blow within 1s so an RA of <0.12 Ω would achieve this, if the earthing method has been in place for along time and not varied it is stable, if the earthing system is below the variations of the changes in ground temperature during the seasons and the ground has settled around the earthing system then the earth should be stable long term.

 

[telectrix]

it might be stable, but the horse can still get out.